1. Field of the Invention
This invention relates in general to vehicle brake rotors and in particular to a brake rotor adapted for use in a vehicle brake assembly and to a method for balancing such a brake rotor.
2. Description of the Related Art
Vehicles are equipped with a brake system for slowing or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefore, are well known in the art.
A typical disc brake assembly includes a rotor which is secured to the wheel of the vehicle for rotation therewith. A caliper assembly is slidably supported by pins secured to an anchor bracket. The anchor bracket is secured to a non-rotatable component of the vehicle, such as the vehicle frame. The caliper assembly includes a pair of brake shoes which are disposed on opposite sides of the rotor. The brake shoes are operatively connected to one or more hydraulically actuated pistons for movement between a non-braking position, wherein they are spaced apart from opposed sides or braking surfaces of the rotor, and a braking position, wherein they are moved into frictional engagement with the opposed braking surfaces of the rotor. When the operator of the vehicle depresses the brake pedal, the piston urges the brake shoes from the non-braking position to the braking position so as to frictionally engage the opposed braking surfaces of the rotor and thereby slow or stop the rotation of the associated wheel of the vehicle.
A typical disc brake rotor is formed from grey cast iron during a sand mold casting process. The rotor includes a generally hat-shaped body, and an outer annular section which are integrally cast as one-piece during the casting process. This kind of rotor is commonly referred to as a “full cast” rotor. In some instances, the rotor is formed with an integrally cast hub, and is referred to as a “uni-cast” rotor.
In the above rotor constructions, the hat-shaped body includes a mounting surface having a centrally located cast-in pilot hole formed therein during the casting process that is machined to size, and a plurality of lug bolt receiving apertures equally spaced circumferentially about the pilot hole. The lug bolt receiving apertures are formed during a subsequent drilling operation.
The outer annular section of the rotor includes two parallel outer surfaces which define a pair of brake friction surfaces. The brake friction surfaces can be cast as a single solid brake friction plate, or can be cast as a pair of brake friction plates disposed in a mutually spaced apart relationship by a plurality of ribs or vanes to produce a “vented” rotor. The brake friction surfaces, as well as other selected surfaces of the rotor including the lug bolt receiving apertures, are typically machined by two “rough” finishing operations followed by a single “finish” machining operation.
High speed rotating components such as rotors spin at high speeds. Any imbalances in the rotor while spinning at high speeds may result in vibration, noise, or premature bearing wear which leads to decreased performance and NVH degradation. Typically the rotors require balancing by the removal of material from the outer diameter of the rotor. The material removed from the outer diameter of the rotor typically includes material from both rotor discs and may also include material from the interconnecting ribs or vanes (i.e., core gap) therebetween. To determine the amount of material to be removed from the rotor, the rotor is rotated at a predetermined speed and is weighed while rotating. An out of balance condition is determined, in addition to the amount of material to be removed. The location and the depth of the material to be removed is also calculated. The rotor is then milled at the desired location for correcting the balance of the rotor.
The material removed by the milling operation is typically removed by a cutter wheel. The cutter wheel is a rotary cutting tool that includes a plurality of cutting blades each spaced circumferentially from one another and extending radially outward above the circumference of the cutter wheel. The location of the material to be removed along the outer circumference of the rotor is brought into contact with rotating cutter wheel. The predetermined amount of material (i.e., calculated length and depth of cut) at the determined location is then removed from the circumference of the rotor.
A disadvantage with removing material from the outer circumference of the rotor is that core gap of the rotor is not a solid surface. Rather the core gap includes vanes or ribs extending between the rotor discs (as described earlier). Typically the vanes in the core gap are not uniform to one another due to inherent inefficiencies in the casting process for the rotors as a result of core wash that is caused by molten material flowing by the core surface. The core wash causes the core opening of the rotor to be larger when measured at the ingate due to the molten cast material (e.g., iron) flowing through the core at the ingate (i.e., a bottom portion of the casting) than at the riser (i.e., top portion of the casting). The calculations made for the amount of material to be removed are based on both discs and the core gap having a uniform structure. But due to the variations in the core gap, inaccurate amounts of material may be removed which may result in an unbalanced rotor which may require additional milling correction operations before the rotor is within the balanced specification.